Research will be continued on projects designed to exploit the finding that a substantial portion of the anti-p-azopheylarsonate (anti-Ar) antibodies of different A/J mice share a cross-reactive idiotypic specificity (CRI). We have isolated this subpopulation, by isoelectric focusing, from individual mice, in amounts sufficient to initiate amino acid sequence analyses. Homogeneity of the heavy chain has been demonstrated. We will now sequence the N-terminal regions of heavy chains from many mice to seek evidence for the occurrence of somatic mutations. The relationship between sequences of anti-Ar antibodies carrying and lacking CRI will be explored in an effort to correlate sequence with specificity, as well as with idiotype. Efforts will e made to determine sequences in the second and/or third hypervariable regions in order to obtain further evidence for homogeneity and to correlate further the amino acid composition of the active site with specificity. Evidence that light chains of antibody with CRI are heterogeneous will be pursued to establish the principle that antibodies of a given idiotype may have identical heavy chains but different light chanins.